Abstract
In the Lesser Antilles region, where the American plates subduct beneath the Caribbean plate, the vast majority of studies focus on the arc and forearc domains, which concentrate most of the volcanic and seismic activity. In contrast, the Lesser Antilles back-arc has long remained little explored and poorly understood from a tectonic point of view. This more than 300 km wide area includes the Aves Ridge, an extinct Late Cretaceous to Paleocene arc emplaced within an oceanic plateau. This ridge, which is about 1 km below sea level, rises above the Grenada Basin that separates it from the active Lesser Antilles arc. The structural relationships between this basin and the adjacent arcs have been much debated, as have the timing and mode of its opening. To answer such questions, this PhD thesis is mainly based on a large multi-channel seismic reflection dataset acquired during the GARANTI cruise in 2017, completely covering the Grenada Basin and the eastern flank of the Aves Ridge. It also includes results from dredges carried out on bathymetric highs of the Aves Ridge. Analysis of these data led to the proposal of a new tectonic model, in which the Grenada Basin opening begins at the end of the Paleocene with NW-SE extension of the Aves Ridge, ultimately resulting in oceanic spreading during the middle Eocene. Today, 3 to 15 km of sediments cover the oceanic crust, which extends across the eastern half of the Grenada Basin between the latitudes of Grenada and Martinique. The asymmetry of the basement and pre-Miocene depocentres suggests that the southern Grenada Basin initially extended further east, probably into the current forearc, before the intrusion of the southern Lesser Antilles arc during the Oligocene-Miocene. This intrusion coincides with a tectonic inversion in the southern basin resulting from the oblique convergence between the Caribbean Plate and the South American continent. Alongside this, special attention was paid to the vertical motions of the Aves Ridge, which is thought to have emerged at the Eocene-Oligocene transition, thus enabling the migration of terrestrial fauna from South America to the Greater Antilles ("GAARlandia land bridge" hypothesis). The discovery of a regional unconformity suggests that the Aves Ridge was part of a land bridge connecting the two Americas in the Cretaceous-Paleocene, approximately 30 Myr before GAARlandia. From the middle Eocene to the middle Miocene, the Aves Ridge underwent relatively slow subsidence, enabling coral reefs to develop on bathymetric highs. Subsidence slowed down in the late Eocene-Oligocene, which could in fact reflect an uplift of the Aves Ridge. However, although some paleo-reefs show evidence of sporadic emersions between the Eocene and the Miocene, there is no evidence of an Eocene-Oligocene emersion in the study area. From the late Miocene, regional geodynamic changes caused an acceleration of subsidence and therefore complete flooding of the Aves Ridge. This study sheds new light on the structure and stratigraphy of the Lesser Antilles back-arc, thus clarifying its tectonic evolution. It is also a first step towards a better understanding of the vertical motions that affected this region during the Cenozoic.